I've been on the road for two weeks with limited internet access.
I did a simultaneous closing on my purchase of the signs in Los Angeles, California, in which I sold three signs to what was then Foster & Kleiser (now Clear Channel). Here's how it worked. We had a simultaneous closing in which Foster & Kleiser wired in the money for the three signs, and I wired in the money for the remainder, and the title company separated who got what (and would have refunded the money to Foster if I had not showed up with my money).
To do this effectively, you have to use two resources:
1) A decent attorney to draft the documents and protect your interests
2) A solid title company to act as referee and protect all parties' interests
Don't try to do this on your own, or in the absence of a title company, as it will probably implode. These type of deals have a huge potential for litigation as you have not two parties to please but three, and their future is all tied together.